A common type of integrated circuit (IC) device is a metal-oxide-semiconductor field effect transistor (MOSFET) that includes a source region, a drain region, a channel region. In high voltage applications, a high voltage MOSFET known as an HVFET (high voltage field effect transistor) maybe used. Many HVFETs employ a device structure that includes an extended drain region that supports or “blocks” the applied high-voltage (e.g., 150 volts or more) when the device is in an ‘off’ or substantially non-conducting state. Conventional HVFETs are commonly formed as lateral or vertical device structures. In a lateral HVFET, current flow, when HVFET is in an ‘on’ state, is horizontal or substantially parallel to a surface of the semiconductor substrate. In a vertical HVFET, current flows vertically through the semiconductor material, e.g., from a top surface of the substrate where the source region is disposed, down to the bottom of the substrate where the drain region is located.
Conventional high voltage IC's often employ a large vertical or lateral HVFET in a configuration wherein the drain of the output transistor is coupled directly to an external pin that may be at a high voltage. The high voltage IC device typically includes a controller circuit that operates on low voltage (0 V-12 V) that is separate from the HVFET, but can be still included in the same high voltage IC. To provide start-up current for the controller circuit of the high voltage IC, a high external voltage may be applied to the external pin. The internal circuitry of the device is typically limit-protected from the high externally-applied voltage by a junction field-effect transistor (JFET) “tap” structure. For example, when the drain of the high voltage output transistor is taken to, say 550 V, the tap transistor limits the maximum voltage coupled to an internal node to approximately 50 V, and also provides a small (2-3 mA) current for start-up of the controller. By way of further background, U.S. Pat. No. 7,002,398 discloses a three-terminal JFET transistor that operates in this manner.
The operating characteristics of a high voltage IC are typically set by a method known as trimming. More specifically, trimming of high voltage IC typically occurs prior to implementation in a useful circuit to adjust certain parameters. More specifically, the process of trimming may involve selectively closing (or opening) one or more electrical elements that indicates to the controller to adjust certain operating characteristics of the high voltage IC. In one example, the electrical elements used for trimming may be zener diodes. During the process of trimming, one or more zener diodes may be off (non-conducting electrical elements). To change the conducting state of a zener element a voltage (>10 V) is typically applied to breakdown the zener. After breakdown of the zener element a current (150-200 mA) passes between the anode and cathode terminals to short the zener element permanently. The cumulative current flowing through the one or more zener elements may be used to program one or more analog parameters. For example, a zener diode may be used to trim or program an analog parameter such as switching frequency in a high voltage IC used in a switch mode power supply. For example, an analog parameter such as switching frequency may be set within a specified tolerance in the controller section of the power IC by short circuiting one or more zener diodes.